Electrochemical machining with fireretardant sodium chlorate compositions

ABSTRACT

The hexavalent chromium content of aqueous chlorate-containing electrolyte solution employed in electrochemical machining or grinding operations may be reduced through reduction to the trivalent chromium state from which it will precipitate as hydrous chromic oxide or by removal in the hexavalent state by precipitation as a barium, lead, zinc, cobalt or copper salt. The removal of trace hexavalent chromium values from a chloratecontaining electrochemical machining electrolytic solution reduces the fire hazard attending the use of chlorate-containing solutions which may accidentally wet an organic material such as an operator&#39;&#39;s clothing and subsequently dry to form an extremely combustible mixture. Likewise, the removal of trace hexavalent chromium values from an aqueous chlorate-containing solution reestablishes the flame-retardant activity of an additive such as sodium metasilicate or sodium hydroxide which activity is reduced in the presence of trace hexavalent chromium ions.

Uitd States Patent Inventors John A. Peterson Niagara Falls; Theodore 11. Dexter, Lewiston, both 01 NY. App]. No. 697,595 Filed Jan. 15, 1968 Patented Oct. 26, 1971 Assignee Hooker Chemical Corporation NiagaraFalls, N.Y.

ELECTROCHEMICAL MACHINING WITH lFlllRlE- RETARDANT SODIUM CHLORATE 143, 14]; 156/19, 18; 252/186, 187; 134/2, l0, l3; 148/6, l4;23/45,56,57

References Cited UNITED STATES PATENTS 2,092,130 9/1937 Lyons, .lr. 204/145 X 3/1938 Williams..... 23/56 7/1957 Comstoclt 204/143 Primary Examiner--Wyman: Daniel E.

Assistant Examiner-J. E. Konopka Attorneys-Peter F. Casella, Richard K. Jackson, Donald C.

Studley, Richard P. Mueller and James F. Mudd ABSTRACT: The hexavalent chromium content of aqueous chlorate-containing electrolyte solution employed in electrochemical machining or grinding operations may be reduced through reduction to the trivalent chromium state from which it will precipitate as hydrous chromic oxide or by removal in the hexavalent state by precipitation as a barium, lead, zinc, cobalt or copper salt. The removal of trace hexavalent chromium values from a chlorate-containing electrochemical machining electrolytic solution reduces the fire hazard attending the use of chlorate-containing solutions which may accidentally wet an organic material such as an operator's clothing and subsequently dry to form an extremely combustible mixture. Likewise, the removal of trace hexavalent chromium values from an aqueous chlorate-containing solution reestablishes the flame-retardant activity of an additive such as sodium metasilicate or sodium hydroxide which activity is reduced in the presence of trace hexavalent chromium ions.

ELECTROCHEMICAL MACHINING it u E- Alkali metal and ammonium chlorates, especially sodium chlorate, are ideal electrolytes for use in electrochemical machining operations. The major problem attending their use resides in their tendency to rapidly oxidize combustible materials. The advantages and problems residing in the use of chlorates as the electrolyte in electrochemical machining operations are summarized in the article appearing in Metal Progress, March 1967, pp. 81-84. For example, when some of the chlorate-containing electrolyte comes into contact with the clothing of the operator, or any combustible material near an electrochemical machining operation, the tendency to initiate tire is great after the wetted material has dried.

By electrochemical machining operations it is intended to include those operations by which a workpiece is machined, milled, or ground so that the metal is removed by an electrochemical process to produce shaped or curved structures. Likewise, those electrochemical processes by which a hole is bored into or through a workpiece or the surface of a workpiece is smoothed is embraced by the expression electrochemical machining operations. The workpiece to be shaped functions as the anode in electrically conducting rela tionship with the cathode die through a suitable electrolyte. Some electrolytes used in electrochemical machining are described in U.S. Pat. No. 2,798,846 to Comstock, issued July 9, 1957 as NaN0,, Na Cr KNO K Cr0 the amines of sodium nitrate, sodium and potassium dichromate, sodium and potassium chlorate and sodium and potassium chlorite. Essentially, the selection of the electrolyte is based upon its conducting properties, its tendency to attack the electrodes, its performance in electrochemical machining operations and its cost. Comstock mentions the rusting of a workpiece and its prevention through the use of a rust-inhibiting agent as the electrolyte.

Electrochemical machining or grinding embraces those procedures which employ a metal or metalloid material as a workpiece from which material is to be removed via electrolytic attack. The workpiece is the anode while the die or grinding wheel is connected as the cathode and the electrical circuit is completed from anode to cathode by the electrolyte (alkali metal or ammonium chlorate). The electrolyte may be directed into the work gap through the cathode if a hollow electrode is employed or the electrolyte may be presented to the work gap by any means known to the art. The solution of electrolyte is held in a reservoir from which it may be fed to the work gap and back to the holding reservoir. Sludge which forms in the electrolyte solution will slowly settle out in the holding reservoir yielding a thickened slurry which way be removed by filtration.

Sodium chlorate is an excellent electrolyte for electrochemical machining operations. Sodium chlorate solutions used as the electrolyte in electrochemical machining are typically aqueous solutions containing about 350 grams per liter Na,Cl0 A typical solution is one containing 3 pounds of sodium chlorate per gallon, which is equal to 359 grams per liter. Hence, a typical electrochemical machining electrolyte may be considered to be an aqueous solution containing from about 300 to 400 grams sodium chlorate per liter. The used electrolyte solutions, usually of pH above about 6 to 7, contain precipitated hydroxides of the metal removed from the workpiece. For example, where the workpiece contains iron, nickel or chromium, the corresponding iron, nickel or chromium hydroxides are formed.

In the electrochemical machining of chromium-containing alloys, some soluble sodium chromate is introduced into the sodium chlorate electrolyte. The introduction of trace amounts (normally between about 0.02 to 2.0 grams as Cr per liter) of sodium chromate into the electrolyte actually increases the fire hazard attending the chlorate and its action on combustible organic material. Hence, any flame-retardant ad- Lit ditive will have to function in the presence of at least low concentrations of dissolved Crll." ions. it is also important that any additive must not change the characteristics of sodium chlorate in its operation as an electrolyte. However, it is most desirable that an additive both act as a flame retardant and improve, or not impair the properties of sodium chlorate as an electrolyte.

Actually, in a standard testing procedure, where the burning time of a cotton twill strip 1.5 inch wide and 52 inches in length is determined, the burning time for untreated cloth is about 36 seconds, whereas the bu'ming time for the cor responding cloth which has bee dried after impregnation with about 1 gram of sodium chlorate solution per gram of cloth under conditions of controlled humidity at room temperature, will vary from about 6 seconds to about 12.6 seconds, based upon the relative humidity of the drying atmosphere of 20 percent for the former and about 52 percent for the latter. As the amount of heitavalent chromium present with the chlorate increases, the buming time for the test cloth dried at 20 percent relative humidity will increase to 4.1 seconds at 0.05 gram Cr per liter; to 3.2 seconds at 0.1 gram Cr per liter; to L9 seconds at from 0.5 to 2.0 grams Cr per liter.

it has recently been discovered that flame-retardant additives may be included with an alkali metal or ammonium chlorate as the electrolyte solution in electrochemical machining and grinding operations. The flame-retardant additives such as sodium metasilicate and sodium hydroxide greatly reduce the burning time of cotton cloth which has been wetted with a chlorate-containing solution and subsequently dried. However, this flame-retarding influence is somewhat diminished when hexavalent chromium is introduced into the electrolyte solution form oxidized chromium metal found in alloys machined by electrochemical techniques.

It is an object of this invention to decrease the tire hazard attending the use of chlorate-containing solutions when organic matter is wetted by such a solution and dried.

It is an additional object of this invention to reestablish the fire-retardant ability of such additives as sodium metasilicate and sodium hydroxide incorporated into chlorate-containing electrolytic solutions for use in electrochemical machining and grinding operations.

DETAELED DESCRIPTION OF THE INVENTION in accordance with this invention, it has been discovered that the fire-accelerating effect of chlorate-induced combustion created by the presence of trace hexavalcnt chromium ions in an aqueous chlorate-containing solution may be avoided by reduction or removal of the hexavalent chromium 10115.

More specifically, it has been found that the hexavalent chromium ion content, which is introduced into the aqueous sodium chlorate electrolyte solution employed to electrochemically machine a metal alloy containing chromium by oxidation of the chromium metal to the hexavalent chromium state (e.g. Coll may be removed by the regular addition of a reducing agent such as an alkali metal or ammonium sulfide, stannous salts such as SnS0.,, SnCl and the ferrous salts such as FeSO FeCll and Fc(I lt') to the chlorate-chromatc-containing solution to reduce the chromate ion to the trivalent chromium valence state in which state the chromium precipitates from solution as hydrous chromic oxide. The reducing agent must not react with the alkali metal or ammonium chlorate in the aqueous solution under the electrolytic conditions employed (i.e. above pH 6 with from about 300 to 400 grams NaClltl per liter).

An especially applicable and the preferred reducing agent is the ferrous Fe) ion, added to the hexavalent chromium-ioncontaining solution as a soluble salt, such as the chloride, sulfate or nitrate. An alkali may be added in combination with the ferrous salt to prevent acidification of the electrolyte solution through formation of HCl, H 30 etc. coming from the ferrous salt. The overall reaction involved in this process taking place in aqueous solution may be noted as:

The values of n and 2: vary with conditions of the precipitation. The hydrous oxides of i-e and Cr thus formed are the same type of compound as the metal hydroxides or hydrous oxides comprising the sludge normally encountered in electrochemical machining operations and the Cr,0 .xll-H O does not noticeably accelerate the burning of cloth when wetted with the chlorate solution, dried and ignited. The hydrous oxides of Fe" and Cr may then be removed from the electrolytic solution along with the sludge derived from the metal coming from the machining of the workpiece by filtration or some other applicable procedure known to the art. After this treatment, only a few parts per million of hexavalent chromium ions remain dissolved in the electrolyte.

The addition of a reducing agent to the hexavalent chromium-containing electrolyte may be made at any stage of an electrochemical machining operation. The hydrous chromium oxide and hydrous iron oxide will remain in suspension in the aqueous electrolyte along with the metal hydroxides or hydrous oxides formed during the machining operation until they are removed by known methods such as filtration. However, it is preferred to add the reducing agent to the aqueous electrolyte just prior to the sludge removal step in the electrochemical machining or grinding cycle. The reducing agent may be well mixed with the electrolyte to afford the optimum reduction of hexavalent chromium. By this procedure, the hexavalent chromium level may be maintained at a very low level in the chlorate electrolyte solution.

if it is not desirable to employ a reducing agent to remove trace amount of hexavalent chromium ions from the chloratecontaining solution, the chromate ions may be removed through precipitation of an insoluble chromate salt.

in accordance with this second aspect of the invention, the aqueous chlorate-containing electrolyte solution which has been contaminated by Cr ions may be treated with a small excess of the required stoichiometric amount of a soluble salt containing a member selected from the group consisting of divalent barium, lead, zinc, cobalt and copper. The anions forming soluble salts with these cations are for example the chlorides, nitrates, etc. However, it is preferred to add these reagents already formulated into an aqueous solution as a nitrate and introduce the reagent in the electrolyte solution just prior to sludge removal so that the BaCrO.,, CoCr0.,, Pb- CrO ZnCro, or CuCrOrCuO precipitate may be removed with the sludge.

Similarly, a low concentration of one or more of the added metal ions could be maintained in the electrolyte solution at all times to precipitate hexavalent chromium ions as they are formed. Barium is the preferred reagent in this situation because it will remain solubilized in an alkaline electrochemical machining electrolyte EXAMPLES l-V To determine the effect of hexavalent chromium ion removal from chlorate-containing electrochemical machining electrolytes, he following experiments were conducted. in each example, a strip of cotton twill 6 feet by LS inch was soaked in the test solution, wrung out and dried under controlled conditions of ZO-percent relative humidity and room temperature. The cloth strips were then hung vertically and ignited at the bottom with a 0.5-inch flame (small source). The time required to burn (or char) a 52-inch section of cloth was determined. in its original state, without treatment, this standard cloth burns in 36 seconds. Hence any treatment of the chlorate-containing electrolyte solution which will prevent the rapid burning that is produced by an alkali metal or ammonium chlorate is an effective fire-rctardant treatment. in each test, the cloth strip was loaded with about 1 gram of solution per gram of cloth. Each solution contained 400 grams sodium chlorate per liter. This concentration approximates the usual concentration of chlorate used for electrochemical machining (3 pounds per gallon=359 grams per liter).

EXAMPLE I The burning time for cloth impregnated with unused electrolyte solution containing 400 grams NaClltl per liter and no hexavalent chromium was 6.0 seconds. After this electrolyte was used in electrochemical grinding of 304 stainless steel containing about ZO-percent chromium, the chromate concentration of the aqueous electrolyte was about 0.5 gram of chromium per liter and the burning time for cloth impregnated with the resulting solution was 4.1 seconds. When the chromium content of the used electrolyte rose to 0.1 gram per liter the burning time was 3.2 seconds; at 0.5 gram chromium per liter a burning time of L9 second was observed.

EXAMPLE ll An electrolyte containing 400 grams NaCiiti per liter and 12 grams NaOll-i per liter as a flame retardant was employed as the electrolytic solution for an electrochemical grinding operation. The resulting solution contained 0.l gram chromium per liter as CrOJ and resulted in a cloth-blaming time of 12.5 seconds. To 1 liter of the used solution was added 20 milliliters of an aqueous solution containing 0.97 gram FeSO, l 10 percent of theory) and 10 milliliters of an aqueous solution containing 0.34 gram of NaOh. After removal of the precipitate, the chlorate-containing solution retained less than 10 parts per million of chromium and gave a burning time of 70 seconds.

EXAMPLE ill An electrolyte solution of 400 grams NaCiti, per liter and 51 grams per liter of Na- SiO as flame retardant was used for electrochemical grinding as in example ii. The final liquid phase acquired 0.5 gram chromium per liter as soluble chromate. As in example ll, solutions containing 4.85 grams of FeSo, and 1.7 grams of NaOH were added simultaneously but separately. The treated liquid contained less than 10 parts per million chromium and resulted in a cloth-burning time of 250 seconds as compared to a burning time 6.9 seconds for the solution prior to chromium removal when ignited with a 4- inch gas flame (large source).

EXAMPLE IV Under the conditions specified in example ll, but without addition of FeSo, or NaOlH, 0.338 gram of Ba(OH),, as a 5- percent aqueous solution was added to 1 liter of the used chlorate solution. BaCrO, precipitated leaving less than l0 parts per million of chromium in the liquid phase. The burning time of cloth impregnated with the resulting solution was improved to 70 seconds. The amount of Ba(0ii) added in this example is percent of the amount theoretically needed to precipitate the dissolved hexavalent chromium.

EXAMPLE V Under the conditions specified in Example iii, but without addition of Fest) and NaOl-l, 3.26 grams of PNNOQ, as a 20- percent aqueous solution was added to 1 liter of the used electrolyte. PbCrO precipitated and was removed to leave a mother liquor containing less than 5 parts per million of dissolved chromium. The burning time for cloth impregnated by the remaining solution was 250 seconds. The amount of Pb(NO added in this example is 105 percent of that theoretically needed to precipitate the dissolved hexavalent chromium.

Having disclosed the invention, it will become apparent to those skilled in the art that obvious modifications may be made which do not depart from the true spirit of this contribu tion. For example, reducing agents other then ferrous salts may be employed to convert hexavalent chromium ions to the trivalent state. For example, any reducing agent may be employed that does not reduce the sodium chlorate or any flameretardant additive present in the electrolyte. Likewise other means for removing hexavalent chromium from the aqueous chlorate-containing solution may be used such as various inorganic salts which form insoluble chromates. Therefore, the examples herein presented are intended to be illustrative of the inventive concepts rather than limitations upon the actual scope of this contribution.

What is claimed is:

l. In an electrochemical machining process in which a metal is removed electrochemically from a chromium-containing workpiece by the application of electrical current from a cathode die to the workpiece anode through an aqueous solution containing an electrolyte selected from the group consisting of an allrali metal chlorate and ammonium chlorate, the improvement which comprises reducing organic material combustibility by removing hexavalent chromium ions from said aqueous chlorate-containing solution by introducing into said solution at least one member selected from the group consisting of the soluble salts of divalent Fe, Sn, Ba, CO, Pb, Zn, Cu, an alkali metal sulfide and ammonium sulfide and removing the precipitate.

2. The process of claim l in which said hexavalent chromium is reduced to trivalent chromium by the introduction into said aqueous chlorate solution oil at least one member selected from the group consisting of an alkali metal sulfide, (NllmS, FeCll,, FeSo Fe(NO SnSo and SnCll 3. The process of claim 1 in which: said hexavalent chromium is reduced by the ferrous salt which is introduced into said chlorate solution with sufficient alkali to neutralize any generated acid.

4. The process of claim It in which said hexavalent chromium is precipitated from said aqueous chlorate solution by the introduction of at least one member selected from the group consisting of soluble salts of divalent barium, lead, zinc, cobalt and copper.

5. The process of claim 1 in which said soluble salts are selected from the group consisting of chlorides and nitrates.

s. The process of claim 4i in which said soluble salts are nitrates.

l W l I21 =0! 222 5 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, ,3 Dated October 26, I971 John A. Peterson and Theodore H. Dexter It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 2, change "CrO to read ---Cr0 j line 12, the word "bee" should read ---been---; line 3, the word "Form" shoul read line 57, change "(e.g. Cr I to read --(e.g. CrO

Column line 20, change "(IrO to read ---CrO,

Signed and sealed this 13th day of June 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR.

ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents 

2. The process of claim 1 in which said hexavalent chromium is reduced to trivalent chromium by the introduction into said aqueous chlorate solution of at least one member selected from the group consisting of an alkali metal sulfide, (NH4)2S, FeC12, FeSo4, Fe(NO3)2, SnSo4 and SnC12.
 3. The process of claim 1 in which said hexavalent chromium is reduced by the ferrous salt which is introduced into said chlorate solution with sufficient alkali to neutralize any generated acid.
 4. The process of claim 1 in which said hexavalent chromium is precipitated from said aqueous chlorate solution by the introduction of at least one member selected from the group consisting of soluble salts of divalent barium, lead, zinc, cobalt and copper.
 5. The process of claim 1 in which said soluble salts are selected from the group consisting of chlorides and nitrates.
 6. The process of claim 4 in which said soluble salts are nitrates. 